This invention relates to data communications and more particularly to measurement of shield continuity in structured datacomm cable systems.
Structured datacomm cabling systems fall into the two categories of shielded and unshielded. Both employ twisted-pair 100 ohm differential impedance transmission lines for signal transmission. The most common cable type contains (4) twisted pairs within a cable jacket. Unshielded (UTP) cable relies on the intrinsic balance of the twisted pairs to reduce EMI and inter-pair crosstalk to acceptable levels, while shielded cable (STP) also includes metallic shielding to further reduce inter-pair and/or inter-cable emissions and crosstalk. The shielding can be around individual pairs, or around the 4-pair bundle, adjacent to the cable jacket.
For STP cabling, defective shield continuity is an important factor that can seriously degrade emissions performance, consequently causing unacceptable cable to cable crosstalk coupling, and EMI problems. For example, cable to cable crosstalk, commonly known as alien cross talk (AXT) is typically not an issue in well shielded terminated cables, but if there is a shield discontinuity, it can be an issue. AXT is especially troublesome for the 10 Gigabit Ethernet over Twisted Pair application, and is a parameter with specified performance requirements. Shield discontinuities, usually opens, can easily be created at connecting hardware by poor workmanship during the manual process of attaching plugs and jacks to cable stock.
DC Verification of shield continuity is possible by measuring the resistance at one end of a link between the shield and a twisted pair, with a loopback connection between shield and pair at the other end. A low resistance indicates good shield continuity. This method, however, often gives false passing results of faulty links, because the shield is typically connected to earth ground at each end of the link. These earth ground connections provide a ‘sneak’ path for DC current that bypasses the open link shield, yielding a low resistance value. FIG. 1 illustrates the DC path through ground that causes this false shield continuity passing result.
The twisted pairs in datacomm cabling systems are normally designed to carry differential mode signals, to minimize emissions and crosstalk. Differential mode is characterized by equal and opposite signal voltages on each conductor, relative to shield. However, a shield open cannot be detected by differential mode measurement methods, since no signal current is returned through the shield, which is thus essentially invisible to the measurement.